In a system in which there exist a plurality of variables (called control quantities) necessary for controlling an object to be controlled, when quantities used to operate the object to be controlled (called operation quantities) are regulated in various manners in order to determine a control quantity, other control quantities may be influenced thereby. This phenomenon is called in general "interference of control quantities" and decreases the precision and the stability of the control.
For example, in a control system for a rolling mill, supposing that control quantities are the plate outlet thickness of the rolled material and the forward tension and operation quantities are a roller position reference or instruction and a rolling speed instruction of the rolling mill, when an operation is carried out according to the roller position instruction and the rolling speed instruction in order to control the plate outlet thickness to be a predetermined value, the forward tension, which is the other control quantity, may be interfered with, thereby being varied.
Heretofore, in order to prevent such an interference, a method has been proposed (JP-A-59-17920, JP-A-59-202108) by which mutual interference between different control quantities is compensated for in advance by varying inputs in a multiple variable control system having three inputs (operation quantities) and three outputs (control quantities) (a control system for a rolling mill). In these prior art techniques, mutual interference between control quantities is eliminated so as to achieve the relation of 1-input-1-output.
Apart therefrom, JP-A-58-207103, JP-A-59-226903. JP-A-61-131103 and JP-A-62-144811 can be cited as prior art techniques relating to this invention.
According to the prior art examples described above, mutual interference is eliminated for all the control quantities. According to such methods, in the case where the scale of the object to be controlled is enlarged, i.e. in the case where the number of control quantities increases, the amount of operations necessary for eliminating the interference becomes enormous. For example, if the number of lines or rows is multiplied by in a matrix operation carried out in the process for eliminating the interference, the number of calculations required for each matrix operation is multiplied by n.sup.2. This gives rise to disadvantages that the device necessary for the control is enlarged, that delay in the control time takes place, which makes it impossible to effect control in real time, etc.
Further, since particular characteristics of the object to be controlled were not taken into account and the elimination of the interference was effected equally for all the control quantities, there was a problem that it was difficult to effect the control without performing useless operations and with a high efficiency.
Further, the prior art examples described above had another problem that they consist exclusively in decomposing an n-input-n-output system into 1-input-1-output systems and therefore they are incapable of coping with an n-input-m-output (nm) system.
Furthermore, in the prior art examples, there is disposed, apart from a driving system control device, another device, called a compensation operating device, having a function different from that of the driving system control device in order to eliminate mutual interference among the plate thickness control, one shape control and the tension control of the rolled material. It was found that, in the case where it is tried to control a same object to be controlled by means of a plurality of control means having different functions in this way, an undesirable interference reaction takes place mutually among these control means, which gives rise to an inconvenience that the desired object may not be achieved satisfactorily.
That is, taking as an example a case where a plurality of control means having different functions are non-interference control means and operation quantity control means as in the prior art example, state quantities are varied by performing certain operations on the state quantities of the object to be controlled by means of the non-interference control means in order to eliminate interference among control quantities. After that, when operation quantities are calculated by means of the operation quantity control means (e.g. optimum control means) on the basis of the state quantities thus varied, the varied state quantities, which have been varied to eliminate the interference, are regarded as control deviations, and the operation quantity control means are operated so as to annul these deviations. In other words, the control is effected so as to reduce the deviations without distinguishing between state quantities which have been varied by external perturbations, and state quantities which have been varied by the non-interference control. Here, control deviation means the difference between the final target value of the control and the actual value of the control.
As a result, the desired control object is not achieved because working points of the operation quantity control means vary, unexpected control is effected, etc.